In order to form films that constitute a semiconductor device in a semiconductor process, films are formed on a substrate by use of a film-forming apparatus such as a plasma CVD apparatus. At this time, films to be formed are attached not only onto the substrate but also onto an inner wall of a processing chamber of the film-forming apparatus. The films attached to the inner wall of the processing chamber are further deposited thereon each time films are formed on the substrate, and are peeled off if they are left intact and may cause particles. Accordingly, plasma cleaning is periodically performed to remove the unnecessary films attached to the inner wall. Moreover, in order to suppress contaminants and to stabilize the process, a film formation process for a predetermined amount is usually performed with no substrate while formed under the same conditions as film formation on the substrate (seasoning).    Patent Document 1: JP-A 11-16845